There is substantial growth in the demand for new wastewater treatment technologies that is being driven by population growth and increasing volumes of wastewater produced, tighter wastewater quality regulations, increasing cost of clean water and water shortages, awareness for the protection of clean water sources and replacement of aging wastewater systems. Industries are specifically being forced both by tougher discharge standards and cost pressures to eliminate their recalcitrant wastewater pollutants prior to discharge, and to adopt on-site water reuse and recycling systems to avoid rising water supply and effluent discharge costs. The requirement is for cost-effective, sustainable water treatment technology that does not require the addition of chemicals and does not produce secondary pollution, is compliant with stringent water quality standards, and has minimal operational and maintenance requirements.
Industrial wastewater can contain organic compounds, many of which are toxic, persistent and resist conventional biological and chemical wastewater treatment. The best approach to treat recalcitrant wastewater is by non-chemical oxidation techniques that can mineralize the pollutants and reduce the organic load and toxicity of the waste, such as electrochemical oxidation. Electrochemical oxidation is sustainable, safe and has a high treatment efficacy eliminating a wide variety of pollutants such as persistent organic pollutants, dioxins, nitrogen species (e.g. ammonia), pharmaceuticals, pathogens, microorganisms, a majority of priority pollutants and pesticides. There are two main approaches to electro-oxidation of pollutants in wastewater. The first is to oxidize pollutants by indirect electrolysis, generating a redox reagent in situ as a chemical reactant. The mediator can be a metallic redox couple or a chemical reagent (e.g. chlorine, ozone, peroxides). These processes require the addition of a large amount of chemicals and/or feed oxygen, and produce secondary pollution leading to additional costs for the disposal of the treated wastewater and operation and maintenance of the process. The second approach is to use direct electrochemical oxidation, where the organic pollutants are oxidized on the anode surface.
A variety of cell configurations that include flow-through parallel plates, divided chambers, packed bed electrodes, stacked discs, concentric cylinders, moving bed electrodes and filter-press have been developed for direct electrochemical wastewater treatment. However, common to all these electrochemical cell configurations is poor operational efficiency leading to high energy consumption. The wastewater is utilized as electrolyte, and in the case of divided cells, both anolyte and catholyte. Due to very low ionic conductivity of wastewater though, the addition of a supporting electrolyte is required to improve the cell efficiency and obtain reasonable cell voltages. This generally results in salt, base and/or acid concentrations that exceed allowable pollutant discharge limits thereby adding to the cost for both the disposal of the treated wastewater and the balance of plant costs of liquid electrolyte handling. Large electrode gaps and low surface area electrodes are also contributors to efficiency losses and increased energy consumption. Slow mass transport in the pores of the porous beds, non-optimized catalyst materials with poor reaction kinetics, high electrode overpotentials, and catalysts with low over potentials for side reactions (e.g. oxygen evolution) also contribute to lower performance and efficiency losses. The use of cell component materials which passivate quickly and increase cell resistivity and instabilities, contribute to efficiency losses. Operating conditions also contribute to efficiency losses. With high mass and ionic transfer losses, at nominal operating current densities, the voltages are too low such that incomplete destruction of organic contaminants occurs and an organic film blocks catalyst sites reducing performance and requiring the use of cell reversal techniques to clean the electrode surfaces.
For instance, published PCT application WO9901382 discloses an electrolytic cell method and apparatus for the decontamination of fluids. The system advantageously comprises means for adding one or more chemical substances into the fluid to be treated (e.g. an acid, carbon dioxide, an alkali, hydrogen peroxide, or a salt.) In another example, Andrade et al., in J. Haz. Mats. 153, 252-260 (2008), disclose the use of a divided electrolytic cell to treat model phenol wastewater. A supporting electrolyte of sulfuric acid was required.
To eliminate the requirement for supporting electrolyte addition, various methods have been developed that reduce the electrode gap in single compartment electrochemical cell configurations. For example, U.S. Pat. No. 6,328,875 discloses the use of a porous anode allowing wastewater to penetrate through the anode to flow through the capillary inter-electrode gaps. However, the energy consumption was still high when run without a supporting electrolyte. As with all single chamber electrochemical systems, hydrogen is simultaneously produced and wastewater constituents are reduced on the cathode, which consume much energy. Fouling of the cathode commonly occurs from these reaction products, decreasing the cell efficiency and leading to increased energy consumption. Another problem encountered in single chamber systems during oxidation is the production of intermediate compounds. These compounds are reduced at the cathode and are then reoxidized at the anode decreasing cell efficiency and increasing energy consumption.
An approach to eliminate the requirement for addition of a supporting electrolyte addition is to use a solid polymer electrolyte (SPE) in the electrolytic cell. SPE technology has been developed for other purposes including the production of hydrogen by water electrolysis and of energy using polymer electrolyte membrane fuel cells. For instance, in the system disclosed in WO03093535, dehalogenation of halogenated organic compounds and destruction of nitrates is conducted on the cathode by electrochemical reduction. In this configuration, the anode and cathode compartments are divided by an ion exchange membrane, and an anolyte and halogen-containing catholyte are passed through their respective chambers. Although the system operated without supporting electrolytes, in order to operate at low current density (high cell efficiency), a supporting electrolyte was required in the anolyte and/or catholyte. Murphy et al. in Wat. Res. 26(4) 1992 443-451 used a SPE electrolytic cell to treat wastewaters with low or negligible supporting electrolyte content. The wastewater was re-circulated through both the anode and cathode. The energy consumption was very high however, and was attributed to low rates of phenol oxidation and side reactions, primarily oxygen evolution from water. J. H. Grimm et al. in J. Appl. Elect. 30, 293-302 (2000) used a SPE electrolytic cell to treat model phenol containing wastewater. The wastewater was pumped through the anode and cathode chambers in series. The energy consumption however was also high for phenol removal, which was attributed by the authors to the loss in current efficiency due to side reactions such as oxygen evolution. Further, A. Heyl et al., in J. Appl. Electrochem. (2006) 36:1281-1290, investigated a range of SPE electrolytic cell configurations at higher temperatures to de-chlorinate 2-chlorophenol model wastewater. In all cases, the wastewater was pumped across the membrane from either the cathode or anode to the opposite chamber through perforations in the membrane or by assisted electro-osmotic drag of treated membranes. The energy consumption was found to be impractically high for the untreated membrane, lower for the chemically treated membrane, and lowest for the perforated membrane. However, the best mineralization was obtained with anodic oxidation first followed by cathodic reduction with higher energy consumption. Still further, another approach for treating low conductivity wastewater without the use of supporting electrolytes was disclosed in WO2005095282. The system used a solid polymer electrolyte sandwiched between anode and cathode electrodes place in a single chamber of low conductivity wastewater. The energy consumption for pollutant mineralization of this setup was high due to the high voltages required.
Systems have also been developed in the art to reduce the cost of producing hydrogen by electrolysis by integrating electrolytic treatment of wastewater therewith. The electrolytic cells involved can use anolytes containing organic pollutants. For instance, Park et al., in J. Phys. Chem. C. 112(4) 885-889 (2008), used a single chamber cell to treat aqueous pollutants and produce hydrogen. As with all single chamber systems, a supporting electrolyte was required. The hydrogen generated was contained in a mixed product gas that required further treatment to recover usable hydrogen. Similar single chamber configurations were disclosed by T. Butt & H. Park in WEFTEC 2008 Conference Proceedings and by J. Jiang et al. in Environ. Sc. & Tech. 42(8), 3059 (2008). Divided cell configurations were disclosed for instance in WO2009045567 and by Navarro-Solis et al. in I J Hydrogen Energy 35 (2010) 10833-10841. The preceding systems all involved the use of additional supporting electrolytes. Systems without supporting electrolytes have also been disclosed for instance by F. Kargi in I. J. Hydrogen Energy 36 (2011) 3450-3456.
Systems using a solid polymer electrolyte based electrolytic cell have also been disclosed in the art to generate hydrogen and to treat wastewater. For instance, U.S. 65/333,919 discloses a method for electrolysis of an aqueous solution of an organic fuel. In this system, permeation of unreacted methanol to the cathode (fuel crossover) takes place and causing high cathode overpotentials and requiring the addition of a hydrogen gas cleaning operation. Further, E. O. Kilic et al. in Fuel Proc. Tech. 90 (2009) 158-163 disclose a system to treat formic and oxalic acid and generate hydrogen. However, the specific energy consumption was high due to the higher current densities required.
Depending on the upstream municipal or industrial process, different particulates and suspended solids may be in the wastewater stream that can affect the performance of electrolytic cells. Suspended solids in wastewater may be organic or inorganic, depending upon the process from which they originate (Industrial Wastewater Management, Treatment, and Disposal, Manual of Practice No.FD-3 (3rd Ed. WEF, 2008). These solids are classified based on size and removal technique. Total suspended solids (TSS) are determined by filtering a sample through a defined filter medium, drying it in an oven, and then determining the residues' weight. For example, in the oil and gas industry, one would typically find particulates and suspended solids derived from oil, high molecular weight and insoluble organics and hydrocarbons, sulfides, hardness, formation solids, corrosion and scale products, and waxes in the wastewater stream. In another example, such as the textile manufacturing industry, one would typically find particulates and suspended solids derived from dye, organics, and non polar compounds in the wastewater stream. In yet another example, such as landfill leachates, one would typically find organics, hydrocarbons, oil, hardness, salts, metals, and non polar compounds. Prior art wastewater treatment systems typically employ a filtration means upstream of the electrolytic cell to filter out such particulates and suspended solids. However, such additional filter systems add a level of system complexity and cost to the overall wastewater treatment system.
Notwithstanding the substantial developments in the art, there remains a continuing need for more efficient and cost effective methods for wastewater treatment. The present invention addresses this need while additionally providing other benefits as disclosed herein.